The invention relates to the playback of video signals and, more particularly, to the control of a scan tracking head jump process with field accuracy during stop and selected variable playback processes, utilizing tape position information derived from a tape timer.
The field accuracy of a video tape recorder in modes other than the play mode, is affected by the properties and eccentricities of a controllable scan tracking head system. For example, when the video tape recorder operates at selected velocities or in stop mode, the tracking offset due to the interchange problems between recording tapes and video tape recorders, will produce regions of the recording tape that will be miss-labeled because the scan tracking head has made its jump prior to the indication of a field transition by the tape timer, or it makes its jump sometime after the tape timer indicates a field transition. That is, in tracking systems that do not include vertical interval time code, or tape timers correlated with the color field identifying inputs, the associated video tape recorder fails to include means which insures that the scan tracking head jump decisions are made precisely at the occurrence of a field-to-field transition. Thus the tape timer may identify one field as the video signal output when, in fact, the head may be tracking a portion of a preceding or subsequent field.
Such a condition is particularly disadvantageous when determining, for example, an insert or assemble edit, where it is imperative that the edit be made precisely at the selected field. In type C recorders, and even in the latest digital video tape recorder systems, editing is often done by first marking the field of desired content as identified visually via a video monitor and by the tape timer. Very often this is done at velocities other than play to ease the decision process. Then the edit is rehearsed and/or made using the same tape timer readout. However, due to the fact that the track jump decision made by the track jump logic of the scan tracking head is responsive only to its elevation, at the precise moment of the edit marking the tape timer may have been indicating an adjacent, improper field rather than the proper field being shown by the video monitor. That is, due to the inconsistency in the head jump control, there is no assurance that the desired field marked for the edit is the same as the field labeled by the tape timer. This condition is undesirable when selecting edit points.
Typical of a system for overcoming the above mentioned problem of incorrectly identifying and/or parking at an incorrect field, such as when searching to an edit entry point, is a system such as used, for example, in a conventional type C or digital video tape recorder. In effect, such a system provides means for holding, or advancing, the reporting of the tape position in response to the control of the scan tracking head from one field to another, while the algorithm for centering the head is maintained at a fixed nominal value. That is, the system basically extracts off-tape information and particularly, extracts the color field identification signal such as generated by a conventional color field 1 detector circuit. With this information available, the field-to-field transitions are known, and the system only allows the tape timer to indicate a new field when the head jump control initiates the signal to advance the tape timer. Therefore, the head jump decision, and thus the field transition appears simultaneous with a field transition of the timer.
In a vertical interval time code (VITC) system, the time code is taken from off-tape and, in effect, from the helical scan mechanism. As the helical scan rotates, it advances to a new field whereby the system reads and reports the occurrence of a new field. The information is obtained during the vertical sync interval and thus is available for updating the time code prior to its display during active video. Such systems of course require that the vertical interval time code be available and correlated with longitudinal time code.
The present invention overcomes the disadvantages of the systems of previous mention, while providing a technique and apparatus for coinciding the scan tracking head movement command with the transition between fields, that is, with the indication of a new field by the tape timer.
To this end, the algorithm for centering the scan tracking head is not left centered at nominal in conventional manner, but continuously is adjusted in response to the tape timer's position within the field. That is, the centering of the scan tracking head is biased forward (or down) when the tape timer indicates that the tape position is early in the field. Or the centering is biased back (or up) when the tape timer indicates the tape position is late in the field. In effect, the scan tracking head is given a shifted scan tracking elevation envelope, which is skewed relative to the nominal center elevation in response to tape position information. The resulting expanded field elevation envelope requires a corresponding increase in the amount of head deflection proportional to the tracking error, but within traditional scan tracking capabilities. As a result a head jump decision and thus a field transition always is made coincident with the tape timer's indication of a transition to the next field. Since the head now always is reading out the proper field, there is no disagreement between the field being scanned and the tape timer readout, and thus there no longer is a region of disagreement between the output video and the tape timer readout.
More particularly, a tape timer system is supplied with control track and capstan tach information and also, if desired, with time code information, in generally conventional fashion. The tape timer system in turn generates a tape timer output signal indicative of tape position in units of hours, minutes, seconds, frames, fields and capstan tachs. A capstan tach residual signal derived from the capstan tach signal of the tape timer output signal, together with a tape velocity signal, is used in a head centering algorithm of the invention, to provide a head centering signal whose value continuously is modified in response to the tape position as represented by the capstan tach residual signal, as well as the tape velocity signal. At such time as a field-to-field transition occurs, and only at such time, the head is allowed to transition to obtain the desired scan track. In effect, the scan tracking head centering is biased with a continuously modified value, which value is proportional to how far through the field the tape has traveled. By way of example only, the algorithm is expressed herein as; centering=1/2f-capstan tach residual, if -1/6.ltoreq.velocity.ltoreq.1/6, where f is a field of deflection corresponding to the distance between tracks, and the value 1/6 is relative to normal tape speed around stop. However, the invention may be used in the velocity range of the order of from -1/6 through 2 1/6 normal play speed. If the velocity is not within the selected velocity range, the centering algorithm is centered on zero with .+-.1/2f tolerance in the manner of the traditional system of scan tracking control.
Although the invention generally is described herein in the environment of a helical non-segmented video recording format, it may be used in helical segmented systems and/or in helical instrumentation recording systems other than video, as further discussed hereinafter.